The present invention relates to an actuating unit for a hydraulic brake system which includes a pneumatic brake power booster and a master brake cylinder connected down-stream of the pneumatic brake power booster. The piston of the master brake cylinder confines a hydraulic pressure chamber and is coupled to an annular piston of larger diameter. The annular piston confines a filling chamber which can be connected to the pressure chamber.
A similar actuating unit is disclosed in German patent number 33 17 996. The annular piston of this actuating unit is hollow and is composed of a cylindrical inner wall and a cylindrical outer wall which are connected at one end and which confine an open annular chamber. The cylindrical outer wall of the annular piston is adapted to move through sealing elements arranged in the master cylinder housing into a filling chamber of annular cross-section at the end of the master brake cylinder close to the brake power booster. The filling chamber is arranged radially outward and coaxially relative to the first pressure chamber of the master brake cylinder. The first pressure chamber is bounded by a bottom that forms the primary piston and closes the cylindrical inner wall.
Less favorable aspects of the prior art actuating unit are its considerable axial length and its complicated structure. Another shortcoming of the prior art is that the pressure fluid volume, which on actuation is displaced by the annular piston through a complicated valve into a supply reservoir, is displaced over the entire stroke of the master brake cylinder. Thus, in the event of a slow actuation, the pressure fluid volume flows through the throttle bores provided in the valve into the supply reservoir so that no filling effect is achieved and the actuating travel is not shortened. On the other hand, in the event of a quick actuation, dynamic pressure develops in the filling chamber so that a sealing cup sealing the primary piston is flooded and the pressure in the primary pressure chamber increases until the valve is opened by the dynamic pressure. In this case, however, filling is undefined and depends on the pressure in the filling chamber. In addition, the slowly progressing release action of the actuating unit of the prior art is a disadvantage. The release action progresses slowly because both the primary chamber and the filling chamber are replenished solely through the previously mentioned throttle bores.